In the description and the claims the term “aluminum p-doped” is used. It means p-doped with aluminum as the p-dopant.
The p-doping of silicon by thermal diffusion of a p-dopant like boron into a silicon substrate is well-known. Thermal diffusion is typically carried out using a diffusion source of the p-dopant, for example, gaseous BBr3. The p-dopant may be thermally diffused throughout the entire silicon substrate thus forming a p-doped silicon substrate (p-type silicon substrate). However, it is also possible for the thermal diffusion process to be carried out in a manner which allows the p-dopant to penetrate only into a surface region of the silicon substrate thus forming only a thin p-doped layer with a low penetration depth of, for example, up to 200 nm. Said thermal diffusion process can be supported by masking certain portions of the silicon substrate surface, i.e. those surface areas which shall not receive the p-dopant.
A solar cell is a particular example of a semiconductor.
A conventional solar cell structure consists of a p-type base with a front n-type surface (front n-type region, front n-type emitter), a negative electrode that is deposited on the front-side (illuminated side, illuminated surface) of the cell and a positive electrode on the back-side. Typically, the p-type base with the front n-type surface is p-type silicon with a front n-type silicon surface.
The production of a conventional solar cell typically starts with a p-type substrate in the form of a p-type wafer, typically a p-type silicon wafer. The p-type wafer may have an area in the range of, for example, 100 to 250 cm2 and a thickness of, for example, 180 to 300 μm. On the p-type wafer an n-type diffusion layer, i.e. an n-type emitter, is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the gaseous phosphorus diffusion source, other liquid sources are phosphoric acid and the like. In the absence of any particular modification, the n-type diffusion layer is formed over the entire surface of the p-type substrate. The p-n junction is formed where the concentration of the p-dopant equals the concentration of the n-dopant; conventional cells that have the p-n junction close to the illuminated side, have a junction depth between 0.05 and 0.5 μm.
After formation of this diffusion layer excess surface glass is removed from the rest of the surfaces by etching by an acid such as hydrofluoric acid.
Next, an ARC layer (antireflective coating layer) of, for example, TiOx, SiOx, TiOx/SiOx, SiNx, Si3N4 or, in particular, a dielectric stack of SiNx/SiOx is formed on the n-type diffusion layer to a thickness of, for example, 50 to 100 nm by a process, such as, for example, plasma CVD (chemical vapor deposition).
The front cathode is typically applied by screen printing and drying a front-side conductive metal paste (front electrode forming conductive metal paste), typically a silver paste, on the ARC layer on the front-side of the wafer. The front cathode is typically screen printed in a so-called H pattern which includes (i) thin parallel finger lines (collector lines) and (ii) two busbars intersecting the finger lines at right angle. In addition, a back-side conductive metal paste, typically a silver or silver/aluminum paste, and a back-side aluminum paste are screen printed (or some other application method) and successively dried on the back-side of the wafer. Normally, the back-side conductive metal paste is screen printed onto the wafer's back-side first as two parallel busbars or as rectangles (tabs) ready for soldering interconnection strings (presoldered copper ribbons). The back-side aluminum paste is then printed in the bare areas with a slight overlap over the back-side conductive metal. Firing is then typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front cathode and the back anodes can be fired sequentially or cofired.
The aluminum paste is generally screen printed and dried on the back-side of the wafer. The wafer is fired at a temperature above the melting point of aluminum; in the typical case of a silicon wafer an aluminum-silicon melt is formed and, subsequently, during the cooling phase, an epitaxially grown layer of silicon is formed that is doped with aluminum. This layer is generally called the BSF (back surface field) layer. The aluminum paste is transformed by firing from a dried state to an aluminum back anode. The back-side conductive metal paste is fired at the same time, becoming a conductive metal back anode. During firing, the boundary between the back-side aluminum and the back-side conductive metal assumes an alloy state, and is connected electrically as well. The aluminum anode accounts for most areas of the back anode, owing in part to the need to form a p+ layer. A conductive metal back anode is formed over portions of the back-side (often as 2 to 6 mm wide busbars) as an electrode for interconnecting solar cells by means of presoldered copper ribbon or the like. In addition, the front-side conductive metal paste printed as front-side cathode sinters and penetrates through (fires through) the ARC layer during firing, and is thereby able to electrically contact the n-type layer.
The wafer and the aluminum back anode together form a bimetallic strip which tends to exhibit warpage (bowing), an undesired behavior, particularly undesired if the bowing exceeds a tolerable upper limit.
MWT (metal-wrap-through) solar cells represent a special type of the aforedescribed conventional solar cells. They have another cell design and they are also well-known to the skilled person (cf. for example, the website “http://www.sollandsolarcom/IManager/Content/4680/qfl7/mt1537/mi30994/mu1254913665/mv2341” and the leaflet “Preliminary Datasheet Sunweb” which can be downloaded from that website and F. Clement et al., “Industrially feasible multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency”, Solar Energy Materials & Solar Cells 93 (2009), pages 1051-1055). MWT solar cells are back contact cells allowing for less front-side shadowing than standard solar cells.
The p-type wafers of MWT solar cells are provided with small holes forming vias between the front- and the back-side of the cell. MWT solar cells have an n-type emitter extending over the entire front-side and the inside of the holes. The n-type emitter is covered with a dielectric passivation layer which serves as an ARC layer. Whereas the n-type emitter extends not only over the entire front-side but also over the inside of the holes, the dielectric passivation layer does not and leaves out the inside of the holes. The inside of the holes, i.e. the n-type diffusion layer not covered with the dielectric passivation layer, is provided with a metallization. The metallizations of the holes serve as emitter contacts and form cathodic back contacts of the MWT solar cell. In addition, the front-side of the MWT solar cell is provided with a front-side metallization in the form of thin conductive metal collector lines which are arranged in a pattern typical for MWT solar cells, for-example, in a grid- or web-like pattern or as thin parallel finger lines. The term “pattern typical for MWT solar cells” means that the terminals of the collector lines overlap with the metallizations of the holes and are thus electrically connected therewith. The collector lines are applied from a conductive metal paste and they are fired through the front-side dielectric passivation layer thus making contact with the front n-type surface of the cell.
Like the back-side of a conventional solar cell, the back-side of a MWT solar cell is also provided with a back-side metallization in the form of an aluminum anode. This aluminum anode is in electric connection with conductive metal collector back contacts, whereby the aluminum anode as well as the conductive metal collector back contacts are in any case electrically insulated from the metallizations of the holes. The photoelectric current is collected from the cathodic back contacts and the anodic conductive metal collector back contacts of the MWT solar cell.
The production of a MWT solar cell is quite similar to the aforedescribed production of a conventional solar cell and it starts with a p-type wafer, typically a p-type silicon wafer. Small holes forming vias between the front- and the back-side of the wafer are applied, typically by laser drilling. The holes so produced are evenly distributed over the wafer and their number lies in the range of, for example, 10 to 100 per wafer. Then an n-type diffusion layer (n-type emitter) is formed over the entire front-surface of the substrate including the inside of the holes.
After formation of the n-type diffusion layer, excess surface glass is removed from the emitter surface by etching, in particular, in a strong acid like, for example, hydrofluoric acid.
Typically, a dielectric ARC layer, for example, of TiOx, SiOx, TiOx/SiOx, SiNx, Si3N4 or, in particular, a dielectric stack of SiNx/SiOx is then formed on the front-side n-type diffusion layer leaving out however the inside of the holes and, optionally, also a narrow rim around the front-side edges of the holes. Deposition of the dielectric may be performed, for example, using a process such as plasma CVD (chemical vapor deposition) or sputtering.
Just like the aforedescribed conventional solar cell structure, MWT solar cells typically have a cathode on their front-side and an anode on their back-side. The front cathode takes the form of thin conductive collector lines arranged in a pattern typical for MWT solar cells. The thin conductive collector lines are typically applied by screen printing, drying and firing a front-side conductive metal paste, typically a silver paste, on the ARC layer on the front-side of the cell, whereby the terminals of the collector lines overlap with the metallizations of the holes to enable electric connection therewith. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C.
As already mentioned, the holes of the wafers of the MWT solar cells are provided with metallizations. To this end, the holes themselves are metallized by applying conductive metal paste, typically silver paste, to the holes, either in the form of a conductive metal layer (open holes) or in the form of conductive metal plugs (holes filled with conductive metal). The metallizations may cover only the inside of the holes or also a narrow rim around the edges of the holes, whereby the narrow rim may be present on the front-side edges of the holes, on the back-side edges of the holes or on both. The metallizations may be applied from one single conductive metal paste. It is also possible to apply the metallizations from two different conductive metal pastes, i.e. one conductive metal paste may be applied to the front-side of the holes and the other to their back-side. After application of the one or two conductive metal pastes it is/they are dried and fired to form n-type emitter contacts and, respectively, cathodic back contacts of the MWT solar cell. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The fired metallizations of the holes are in electric connection with the terminals of the thin front-side conductive collector lines.
In addition, a back-side conductive metal paste, typically a silver or silver/aluminum paste, and an aluminum paste are applied, typically screen printed, and successively dried on the back-side of the p-type wafer avoiding any contact with the metallizations of the holes. In other words, the back-side metal pastes are applied ensuring that they stay electrically insulated from the metallizations of the holes prior to as well as after firing. The back-side conductive metal paste is applied onto the back-side as anodic back collector contacts, which may take the form of busbars, tabs or evenly distributed contacts. The back-side aluminum paste (for BSF formation) is then applied in the bare areas slightly overlapping with the conductive metal back collector contacts. In some cases, the back-side conductive metal paste is applied after the back-side aluminum paste has been applied. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front cathode, the metallizations of the holes and the back anodes can be fired sequentially or cofired.